1. Field of the Invention
This invention relates to electrochemical cell electrodes and, more particularly, a method for making said electrodes.
2. Description of the Prior Art
Electrodes for use in electrochemical cells such as fuel cells are well known in the art. One common type of electrode is the gas diffusion electrode. A pair of gas diffusion electrodes is disposed on either side of a compartment containing a matrix which is soaked with electrolyte. A catalyst is disposed on the electrolyte facing surface of each electrode. Hydrogen is fed to the back side of one electrode while oxygen or air is fed to the back side of the other electrode. The gases enter the electrodes and react with the electrolyte in the presence of the catalyst.
Many types of gas diffusion electrodes are described in the literature. One type of gas diffusion electrode comprises a layer of polytetrafluoroethylene (PTFE) mixed with a catalyst supported on carbon particles (i.e., catalyzed carbon), said layer being disposed on the surface of a porous carbon substrate (e.g., carbon paper). The PTFE prevents the electrolyte from filling up the electrode to such an extent that sufficient fuel or oxidant cannot reach the catalyst. A gas diffusion electrode of this type is described in column 5 of commonly owned U.S. Pat. No. 3,857,737 to Kemp et al.
Another type of gas diffusion electrode comprises a carbon paper substrate with a layer of uncatalyzed carbon mixed with PTFE applied to the surface thereof; unsupported catalyst is then applied to the carbon/PTFE layer. An electrode of this type is described in commonly owned U.S. Pat. No. 3,972,735 to Breault.
One technique for applying either the catalyzed carbon/PTFE layer or the uncatalyzed carbon/PTFE layer to a substrate is the direct filtration method. In that method carbon powder (catalyzed or uncatalyzed) and an aqueous PTFE dispersion are blended into an alcohol/water solution to form a co-suspension of carbon and PTFE therein. The suspension is caused to floc, such as by heating or adding a floccing agent. Floccing is the coalescence of the catalyzed or uncatalyzed carbon particles with the PTFE particles. A proper floc is one which achieves uniform size agglomerates of catalyzed or uncatalyzed carbon and PTFE particles and a homogeneous distribution or mix of the particles. After floccing excess liquid is decanted and the floc is applied to the surface of a carbon paper substrate which acts as a filter. Liquids pass through the substrate and a layer of the desired solids mixture (i.e., the floc) remains on the surface of the substrate. The article is dried, compacted, and sintered. If the applied layer does not include catalyst, a separate catalyst application step would be required.
A related technique is the indirect filtration (or filter transfer) method wherein the layer of floc is applied to the substrate by first filtering it onto special filter paper instead of onto the carbon paper substrate. The layer is then transferred from the filter paper onto the carbon paper, dried, compacted, and sintered as in the direct filtration method.
Satisfactory electrodes have been made by both of the foregoing filtration methods; however, these techniques are slow, expensive, and not suitable for production runs wherein hundreds and thousands of electrodes must be made in a relatively short period of time at minimal cost.
It has been our experience that different methods for applying a carbon/PTFE layer usually result in an electrode with different performance characteristics. One reason for this is that the arrangement of carbon and PTFE particles relative to each other in the layer, the precise nature of the particles themselves (such as their surface chemistry), and the uniformity of the layer and imperfections in the layer often vary from method to method. For example, a layer formed by first applying carbon powder to the substrate and then infiltrating the carbon powder with PTFE yields a completely different structure than that formed by preparing a co-suspension of carbon and PTFE, floccing the co-suspension, and applying the wet floc to the substrate.
Thus, it is not a small task converting from an expensive, slow, electrode fabrication method which is known to produce highly satisfactory electrodes, to a faster, more economical production type fabrication process.
Two patents which relate to methods for applying materials to fuel cell electrode substrates and which, therefore, may be of general interest to the subject matter of the present application, are U.S. Pat. No. 3,573,991 Lenfant et al and U.S. Pat. No. 3,591,421 Schultze et al. The former is directed to the use of electrostatic projection for the purpose of forming a variety of layers which constitute an electrode. Electrostatic projection involves imposing a charge on a support, imposing an opposite charge on the particles of a powdered material which is to be applied to the support as a layer, and fluidizing the charged particles of powder above the oppositely charged support. The powder is attracted to the support and coats the support to the thickness which is dependent upon a number of parameters.
Experiments have been conducted using electrostatic projection to apply a dry carbon/PTFE floc to a carbon paper substrate but were unsuccessful due to the inability to apply a proper charge distribution to the floc.
Schultze et al, in column 6 at lines 19-25, indicates that a hydrophobic polymer powder can be distributed within the pores of a porous substrate by moving the substrate through a chamber in which fine particles of the hydrophobic polymer powder are uniformly whirled up and thrown against an exposed face of the substrate while applying an intermittent vacuum to the opposite face. The express object of Schultze et al is to obtain a specified density distribution of hydrophobic polymer within the pores of the substrate. Later heating of the substrate fixes the polymer particles to the walls of the pores as discussed in column 5 at lines 44-53. As will become clear from the description of Applicants' invention as set forth hereinbelow, Applicants' method is not for the purpose of wetproofing the substrate nor does it introduce a hydrophobic polymer into the pores thereof. Rather, it is a method for applying a layer of dry powder comprising carbon and a polymer on the surface of a substrate which is usually already wetproofed.